there r lot of factors dat decide d concept of main line.
here in case of 3 lines of LKO - BSB.
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LKO to FD was d first line to built however LKO - PBH was d first line to reach BSB.
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LKO - PBH - BSB is called d main line. 302km
LKO - SLN - BSB is called d chord line. 284km
LKO - FD - BSB is called d loop line. 324km